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Date: Wed, 20 Jul 2011 16:52:22 +0300 (EEST)
From: Pekka Enberg <penberg@kernel.org>
Subject: Re: possible recursive locking detected cache_alloc_refill() +
 cache_flusharray()
In-Reply-To: <1311168638.5345.80.camel@twins>
Message-ID: <alpine.DEB.2.00.1107201642500.4921@tiger>
References: <20110716211850.GA23917@breakpoint.cc>  <alpine.LFD.2.02.1107172333340.2702@ionos>  <alpine.DEB.2.00.1107201619540.3528@tiger> <1311168638.5345.80.camel@twins>
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Sebastian Siewior <sebastian@breakpoint.cc>, Christoph Lameter <cl@linux-foundation.org>, Matt Mackall <mpm@selenic.com>, linux-mm@kvack.org

On Wed, 20 Jul 2011, Peter Zijlstra wrote:
>>>> just hit the following with full debuging turned on:
>>>>
>>>> | =============================================
>>>> | [ INFO: possible recursive locking detected ]
>>>> | 3.0.0-rc7-00088-g1765a36 #64
>>>> | ---------------------------------------------
>>>> | udevd/1054 is trying to acquire lock:
>>>> |  (&(&parent->list_lock)->rlock){..-...}, at: [<c00bf640>] cache_alloc_refill+0xac/0x868
>>>> |
>>>> | but task is already holding lock:
>>>> |  (&(&parent->list_lock)->rlock){..-...}, at: [<c00be47c>] cache_flusharray+0x58/0x148
>>>> |
>>>> | other info that might help us debug this:
>>>> |  Possible unsafe locking scenario:
>>>> |
>>>> |        CPU0
>>>> |        ----
>>>> |   lock(&(&parent->list_lock)->rlock);
>>>> |   lock(&(&parent->list_lock)->rlock);
>>
>> On Sun, 17 Jul 2011, Thomas Gleixner wrote:
>>> Known problem. Pekka is looking into it.
>>
>> Actually, I kinda was hoping Peter would make it go away. ;-)
>>
>> Looking at the lockdep report, it's l3->list_lock and I really don't quite
>> understand why it started to happen now. There hasn't been any major
>> changes in mm/slab.c for a while. Did lockdep become more strict recently?
>
> Not that I know.. :-) I bet -rt just makes it easier to trigger this
> weirdness.
>
> Let me try and look at slab.c without my eyes burning out.. I so hate
> that code.

So what exactly is the lockdep complaint above telling us? We're holding 
on to l3->list_lock in cache_flusharray() (kfree path) but somehow we now 
entered cache_alloc_refill() (kmalloc path!) and attempt to take the same 
lock or lock in the same class.

I am confused. How can that happen?

 			Pekka

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